1. Field of the Invention
This invention relates to an excimer laser device designed to be used as a light source for reduction projection aligners, for the fine working of materials, for the surface modification of materials, and the like, and in particular to an improvement aimed at stabilizing the beam profile of output laser light.
2. Description of Related Art
When an excimer laser device is conventionally operated using a halogen gas, the gas is consumed during the operation due to the evaporation of the electrode materials and to the chemical reactions involving the structural materials of the laser chamber. The following control measure was therefore adopted in the past to compensate for the reduction in laser output due to the loss of the halogen gas.
Specifically, laser output is obtained by feeding electric energy,. which is accumulated in a capacitor in order to excite the laser, to a discharge space and initiating an electric discharge in a laser medium gas. The laser output (power) is increased, however, when the charging voltage of this capacitor is raised. In the past, therefore, the laser output was stabilized by detecting the output and controlling the value of the charging voltage in accordance with the detection results. This type of control is called "powerlock control" ("POWERLOK" is a registered trademark of Questek, U.S.A.).
Even with this type of control, however, the oscillation efficiency is reduced due to the loss of the halogen gas when the operation is conducted for a long time, and it becomes impossible to maintain the prescribed output unless the charging voltage is gradually increased.
In view of the above, the aforementioned loss of the halogen gas was dealt with in the past by replenishing a constant amount of the halogen gas when the charging voltage rose above a certain prescribed voltage.
This conventional technique is described, for example, in the Official Gazette of Published Unexamined Patent Application of Japan (Japanese Laid-Open Patent Application 3-135089). A technique related to this prior art is also described in U.S. Pat. No. 4,977,573.
The following is a description of an embodiment which, although different from the embodiments described in the aforementioned Official Gazette, is technologically similar and is preferable for implementation. FIG. 23 and FIGS. 24a through 24e are drawings illustrating a preferred embodiment of a method for replenishing the halogen gas.
Specifically, FIG. 23 illustrates the structural components related to gas replenishment for a common discharge-excited excimer laser device, in which case the cylinders used for gas injection comprise a cylinder 1 filled with a halogen gas (F.sub.2, HCl, or the like), a cylinder 2 filled with krypton or other such rare gas, and a cylinder 3 filled with neon, helium, or other such buffer gas.
Laser light that is output by a laser chamber 4 passes through a beam splitter 5, a portion of the light strikes an output detector 6, and the output value E thereof is detected by the output detector 6. The detected output value E is input to a controller 7.
Meanwhile, the pressure (total pressure) PT of the laser gases inside the laser chamber 4 is detected by a pressure sensor 8, and the detected value PT is also input to the controller 7.
The controller 7 controls the charging voltage V of an electric discharge power source 9 on the basis of these detected values E and PT, and also controls a gas supply and exhaust device 10, thus effecting the supply and exhaust control of the laser gases.
The specifics of this case are as follows: gases are fed, for example, in a halogen gas:rare gas:buffer gas ratio of 4:40:2456 (torr), and the target laser output E.sub.c, optimum control charging voltage range V.sub.m (V.sub.min to V.sub.max), and the like are established.
When the operation is started, the controller 7 receives the laser voltage E detected by the output detector 6, the laser voltage E is compared with the target laser output E.sub.c, and control is effected in the following manner: when E&lt;E.sub.c, the charging voltage V is changed by a microscopic voltage .DELTA.V; when E=E.sub.c, the charging voltage V is left unchanged; and when E&gt;E.sub.c, the charging voltage V is lowered by the microscopic voltage .DELTA.V.
In addition, the controller 7 compares the charging voltage V with the maximum value V.sub.max of the charging voltage and controls the operation in such a way that when V&gt;V.sub.max, a prescribed amount of the halogen gas is supplied into the laser chamber 4 from the halogen gas cylinder 1.
FIGS. 24a through 24e are time charts related to the aforementioned control procedures. FIGS. 24a, 24b, 24c, 24d, and 24e illustrate the laser voltage E, charging voltage V, halogen gas partial pressure, rare gas partial pressure, and beam width, respectively. Specifically, when the charging voltage V does not exceed V.sub.max, the charging voltage V is changed (raised in this case) in order to make the laser voltage E constant, and because the charging voltage V reaches the maximum allowable level V.sub.max at times t.sub.1, t.sub.2, . . . , t.sub.6, the halogen gas is injected at each of these times.
Because such control procedures do not involve gas exhaust during the injection of the halogen gas, the partial pressure of the rare gas krypton is kept constant, but an oversupply of the halogen gas gradually builds up as the number of replenishment cycles is increased, ultimately disrupting the gas balance. Specifically, every time the halogen gas is replenished in accordance with the aforementioned conventional technique, the optimum compositional balance of the mixed gases in the laser chamber becomes more disrupted, making it impossible to keep the laser output constant despite repeated attempts to control the charging voltage impressed on the capacitor.
It should be noted that the aforementioned conventional technique, which involves controlling only the laser output to keep it constant, can successfully control the laser output and keep it constant, but does not control the beam profile in any way, and is thus disadvantageous because of the wide fluctuations of the beam profile. A specific example of a beam profile is the beam width in a direction perpendicular to the discharge direction of an output laser; FIG. 24e illustrates the manner in which the beam width W changes over time. As can be seen in this figure, the beam width W undergoes substantial changes in the same manner as does the partial pressure of the halogen gas.
These changes in the beam profile alter the energy density of the laser light illuminating the surface being exposed or processed, resulting in poor exposure quality or processing quality. The electrode shape, operating discharge voltage, gas composition, gas pressure, and the like are factors that change the beam profile.